At 140 ft 2+1⁄2 in (42.74 m) overall, engine and tender, the S1 was the longest reciprocating steam locomotive ever; it also had the heaviest tender (451,840 lb / 205 tonnes), highest tractive effort (76,403 lbf (339.86 kN)) of a passenger steam engine when built and the largest driving wheels (7 feet in diameter) ever used on a locomotive with more than three driving axles.

They also hoped that the new S1 steam locomotive would have a performance equal to their Pennsylvania Railroad class GG1 electric engine and would be capable of hauling a 1,000-ton passenger train at 100 mph.

In July 1936, PRR requested Baldwin Locomotive Works to submit a design for a 4-8-4 engine capable of handling a 2,000-ton train between Colehour and Harsimus Cove.

However, the cooperation between PRR and Baldwin, which proceeded without signing any agreement or contract for the development of the new high-speed duplex engine, didn't go smoothly.

[5] The members of the joint committee were: On 28 April 1937, PRR's Board authorized $300,000 for this experimental high-speed passenger locomotive project.

On 2 June 1937, PRR officially announced the development of the “Pennsylvania Type” high-speed passenger locomotive which would become Class S1.

[4] In August 1941, PRR VP-Western Region James M. Symes, a senior official who turned against the idea of duplex engine in later years, approved the extension of stall no.

The boiler for the S1 was the largest built by the Pennsylvania Railroad; with 660 square feet (61 m2) of direct heating surface and 500 one-inch diameter tubes, the total heating surface area of S1 was 7,746 square feet (719.6 m2); it was 99.3% as massive as the boiler for Union Pacific's 4000-class "Big Boy" locomotives.

[citation needed] The large Belpaire firebox met the Pennsy's standards; its heating surface area included that supplied by seven American Arch circulators.

On the same day, it made its first road test with two cars, running backward to Huntingdon and returned to Altoona at speeds up to 50 miles per hour (80 km/h).

S1 was towed (facing backward) by smaller freight engines like PRR Ils and took a round-about route to the fair site.

At the Fair, the drive wheels operated under the locomotive's steam power[4] ran continuously on the roller platform at 60 mph (97 km/h) for an entire day.

As one of the most important exhibits of the World's Fair, S1 was used by the PRR for various publicity purposes; her image was featured in calendars, stamps, advertisements, brochures, puzzles, etc.

The S1 was so large that it could not negotiate the track clearances on most of the lines of the PRR system, in its brief service life it was restricted to the mainline between Chicago, Illinois and Crestline, Ohio (283 miles/446 km).

[20] Based on photographic evidence, the S1 hauled the Broadway Limited (New York to Chicago) and Liberty Limited (Washington D.C to Chicago) in the first few months of its revenue service, and was then assigned to haul other popular, heavier and commercially successful passenger trains such as The General, The Trail Blazer and The Golden Arrow on this route.

The S1 helped PRR to handle the extreme busy wartime traffic until the end of WWII and paid off its high construction cost within one year.

In terms of tractive effort and drawbar horsepower, the PRR S1 was the most potent reciprocating steam locomotive ever built for passenger service.

Some publications from Germany stated that the S1 was able to reach 120.01 miles per hour (193.14 km/h) in other road tests during the late 1940s when pulling lighter load, but PRR never claimed this record.

On Apr 19, 1941, during an excursion run organized by the Detroit Railroad Club, the S1 reached 105 miles per hour (169 km/h) on the route between Fort Wayne and Chicago.

[22] An article "Riding the Gargantua of the Rails"[23] in the Dec 1941 Popular Mechanics Magazine cites a speed recorded by assistant road foreman Charlie Wappes of the Fort Wayne Division during the S1's test runs at 133.4 miles per hour (214.7 km/h) with a train of 12 heavyweight passenger cars.

In the German trade press and literature from 1945 there was a report of a record run of the S1, citing railroad officials of Interstate Commerce Commission that a speed of 141.2 miles per hour (227.2 km/h) was reached when the engine was trying to make up time for a delayed westbound train, the Trail Blazer.

The engine was claimed to have exceeded 152 mph (245 km/h) on the Fort Wayne-Chicago run, as it was reported that the PRR received a fine for the feat.

[citation needed] The streamlining designer of the train Raymond Loewy himself wrote in 1979: "On a straight stretch of track without any curves for miles; I waited for the S1 to pass through at full speed.

Although the S1 had less than half (47%) its total weight on the driving wheels, its Factor of adhesion was still close to the much more successful PRR Q2, Santa Fe "Northern" 4-8-4s the 2900s, and the renowned N&W J class 4-8-4s.

More than half of its weight was carried by the three-axle pilot (leading) and trailing trucks instead of the drivers, which left the four duplex driving axles susceptible to wheel slippage.

The S1 served between Chicago and Crestline, Ohio for almost 5+1⁄2 years, giving it a longer career than other experimental PRR steam locomotives, such as the Q1 4-6-4-4 and S2 turbine 6-8-6.

Railway Historian and Author Alvin F. Staufer agrees that she (S1) was oversized and thus unable to visit most roundhouses or handle tight curves, but contends: "She was an excellent steamer and gave trouble-free service.

[29] In Oct 1939, when the S1 was still on display in the 1939-40 World's Fair, Baldwin placed a stock order to build a 4-4-4-4 duplex high-speed passenger locomotive as a demonstrator, with bullet nose streamlining by noted industrial designer Otto Kuhler.